Typically when I chamber a round in any gun other than a hunting rifle or carry gun, it gets fired. So I was caught off guard when I cleared my m&p 15 at the range for cease fire and noticed that the primer had been struck (lightly). Out of curiosity, I ran about 5 live rounds into and out of battery and all were dimpled. I have an ar but am far from an "ar guy". I know the firing pin floats freely but my gut tells me that this is not supposed to happen. Any input is welcome. Thanks.

That is normal. The free floating firing pin continues forward when the bolt stops and will strike the primer. It is more likely there is something wrong if you don't see the light primer strikes.
If it bothers you, release the bolt and use the charging handle to slowly let it close and a few taps on the forward assist.

I had the same thing last weekend. I thought the round had been dimpled by nearly charging a second round.

When coyote hunting, I ride the bcg forward to be as quiet as possible and then forward assist into Full battery. Somehow I had chambered a round and when double checking the extractor had never grabbed the chambered round. I then attempted to chamber another behind it.... In my defense it's usually pitch black when I walk in. I thought the second round had dimpled the primer, but it was just a previously chambered round with firing pin strike.

That is normal. The free floating firing pin continues forward when the bolt stops and will strike the primer. It is more likely there is something wrong if you don't see the light primer strikes.
If it bothers you, release the bolt and use the charging handle to slowly let it close and a few taps on the forward assist.

That is what I was hoping to hear. It doesn't bother me as long as it is part of the design. I will continue to slam the bolt home as long as I have no need to worry about sending one down range.

The AR is designed for use with 5.56 mm MILITARY ammo, Machinegun ammo. The military primers are much harder than most commercially available primers. Using commercial .223 ammo you will see the slight dimpling. Rarely does this result in a slamfire (avoid Federal primers as they tend to be the softest).

I have NEVER seen or experienced a slam fire because of this. If it concerns you a bunch, look into a Titanium firing pin. The reduced mass of the Titanium will greatly reduce this tendancy.

__________________
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball

__________________
______
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
________________________________________
US Army 1966-69, VFW Life Member, Retired Geek